The gunman who massacred 26 children and adults in America's worst school shooting killed his victims at close range after forcing his way into the building, officials say.

Medical examiner Dr H Wayne Carver said all the dead, most of them aged just six or seven, had a number of gunshot wounds inflicted with an assault rifle.

It had earlier been reported that 20-year-old Adam Lanza had used two handguns in the rampage at Sandy Hook Elementary School in Newtown, Connecticut.

Mr Carver added that the shooting was the "worst I have ever seen".

Two people embrace after learning of the school massacre

Lieutenant Paul Vance told a news conference that the suspect forced his way into the building after killing a relative, believed to be his mother.

After storming through several classrooms, Lanza, who was wearing a bullet-proof vest, turned a gun on himself.

Video:Doctor: Victims Hit Multiple Times

"It's believed he was not voluntarily let into the school at all, that he forced his way into the school, but that's as far as we can go on that," Lt Vance said.

The painstaking investigation of the crime scene itself is ongoing, and likely to continue for a further two days, he added.

When asked whether any emails or notes had been discovered that could shed light on the gunman's motive, he said "very good evidence" had been found.

"Our investigators at the crime scene, the school and secondarily at the secondary crime scene … did produce some very good evidence in this investigation that our investigators will be able to use in hopefully painting the complete picture as to how and more importantly why this occurred."

Video:Police Audio Of Shooting Unfolding

He said a woman at the school who survived the shooting is doing well and will be a key part of the investigation.

"She is doing fine. She has been treated, and she'll be instrumental in this investigation, as I'm sure you can understand," he said.

Authorities have visited local gun ranges in Connecticut but have found no evidence that the gunman trained for the attack or was an active member of the recreational gun community.

Headteacher Dawn Hochsprung and school psychologist Mary Sherlach both died while attempting to confront the gunman.

Maryann Jacob, who worked in the library, told how she led 18 children to safety by crawling with them to a storage room and waiting for the police to arrive.

She said they barricaded the door with filing cabinets, only opening it when a police officer slid an identification badge underneath.

The town's first selectman Patricia Llodra spoke to reporters on behalf of the community.

"Newtown has suffered an horrendous tragedy, a harm that has broken our hearts," she said. "Our wound is deep because we are a close-knit community, we truly care for each other."

Video:US In Shock Over School Massacre

President Barack Obama used his weekly address to express his sadness in the wake of the shooting.

"We grieve for the families of those we lost. And we keep in our prayers the parents of those who survived. Because as blessed as they are to have their children home, they know that their child's innocence has been torn away far too early," Mr Obama said.

He also hinted at possible gun law reform, declaring that the latest atrocity signals that the country should come together to take meaningful action, "regardless of the politics".

Meanwhile in Alabama authorities say a man opened fire in a hospital, wounding an officer and two employees before he was fatally shot by police.

Birmingham Police Sergeant Johnny Williams said the injuries suffered by the officer and employees are not life threatening.

He said police were called because a man with a gun was walking through St. Vincent's Hospital on Saturday morning.